1. Field of the Invention
The present invention relates to a superconductor thin film and a manufacturing method therefor, and a thin film superconductor device and a manufacturing method therefor.
2. Prior Art
The P-type oxide superconductive material using electron holes as the electric charge transfer carrier in the normal conductive state of La-Sr-Cu-O base, Y-Ba-Cu-O base, Bi-Sr-Ca-Cu-O base, and Tl-Ba-Sr-Cu-O base materials, etc., has not been clarified in detail as to the superconductivity mechanism thereof, but the transition temperature thereof is higher than the temperature of liquid nitrogen. Therefore, an application to various fields of electronics such as quantum interference elements.
These materials show the change from an insulator (semi-conductor) to a superconductor, depending upon the amount of oxygen atoms contained in the crystal, that is, the state of oxidization. In order to obtain a good superconductor material, the improvement of crystallinity and control of oxidization state are required. According to the manufacturing method for the sintered superconductor material obtained so far, a high temperature process above 800.degree. C. in the oxygen ambient and a slow cooling process below 100.degree. C. are required, which necessitates an installation of a high temperature furnace and a long processing time.
Unlike these P-type superconductor materials, materials of such as Nd-Ce-Cu-O base, Nd-Cu-o-F base, etc. have crystalline structure of Nd.sub.2 CuO.sub.4 type and N-type oxide superconductor materials including electrons as electric charge carriers in the normal conductivity state.
The N-type superconductor material like the P-type superconductor material shows the change from an insulator (semiconductor) to a superconductor, depending upon the amount of oxygen deficiency contained in the crystal, that is, the state of oxidization. In order to obtain a good superconductor material, improvement of crystallinity and control of oxygen deficiency (state of oxidization) are required. According to the manufacturing method for the superconductor material so far obtained, a careful annealing process at a high temperature above 800.degree. C. under the vacuum (reduction atmosphere) was necessary.
These new superconductor materials can be prepared only by the sintering process primarily at the current technical level and may be available only in the form of a ceramic powder or block. On the other hand, in case of placing these materials into practical use, although processing into thin films is strongly demanded, it is very difficult to prepare a thin film of good superconductive characteristics according to the conventional technique.
In each of P-type and N-type superconductor thin films so far prepared, the current density in the superconductivity critical condition is small and the time-dependent logarithmic decrease of current density is remarkable, and therefore, it has been considered difficult to realize a superconductor device of high stability and reliability.
In the case of using these p-type and N-type cuprocompound superconductors as a superconductive magnetic memory, various methods have been devised to distinguish 1 and 0. In the case of utilizing superconductor thin film for super conductive magentic memory, according to the conventional proposal, the distinction between 1 and 0 memory states are made by detecting fluxoids replenished in the superconductor material or in the space enclosed by superconductors with a detector to examine the presence or non-presence of fluxoids, and by using a cuprocompound superconductor of high transition temperature, it is possible to operate a superconductive magnetic memory at the liquid nitrogen temperature.
Conventionally, there have been proposed various superconductive magnetic memories using niobic nitride (NbN) of an A15 type two-component compound or niobic germanium (Nb.sub.3 Ge). However, the superconductivity transition temperatures T.sub.c of these materials are 24 K. at the most. Furthermore, as perovskite compounds, there are known materials of Ba-Pb-Bi-O base (Japanese Patent Laid-Open Publication No. SHO60-173885/1985) and there are researched a lot of superconductor elements using materials of this base. However, T.sub.c of this material is relatively as low as 13 K. and it is difficult to place this material into practical use. The high temperature superconductor material has a transition temperature higher than the liquid nitrogen temperature and it is greatly expected to place said material into practical use.
Recently, there is reported a field effect type superconductor transistor using high temperature superconductor thin film (A. Yoshida, H. Tamura, N. Fujimaki, and S. Hasuo, Extended Abstracts of 1989 International Superconductivity Electronics Conference, DE2-2). This superconductor transistor is prepared in the following manner: That is, after forming Ba-Y-Cu-O superconductor thin film on a MgO substrate, by heat-treating at 600.degree. C. in the nitrogen atmosphere, said superconductor thin film is reduced to prepare a thin film of semiconductive characteristics deficient of oxygen, and after a gate insulating film made of BaF.sub.2 and a gold gate electrode are formed on said semiconductor thin film, by exposing them to oxygen plasma, oxygen is doped so as to recover superconductive characteristic in the thin film portions corresponding to the source area and drain area, thus to prepare a superconductor transistor.